1. Field of the Invention
This invention relates to a process for producing substituted or unsubstituted naphthalic acids and acid anhydrides thereof by oxidation of corresponding acenaphthenes whose naphthalene nucleus may have one or two substituents selected from the group consisting of a halogen atom, a sulfonic acid group or sulfonate group, and a nitro group.
2. The Prior Art
In the oxidation of acenaphthenes into corresponding naphthalic acids, it is the common practice to use chromates, bichromates, permanganates or nitric acid as oxidizing agents. However, the use of these oxidizing agents involves several disadvantages in that they are expensive and there is a possibility that environmental pollution may be caused by contamination with the heavy metals or with nitrogen oxides.
Several processes of producing naphthalic acid by oxidation of acenaphthene per se are known including processes as described in U.S. Pat. No. 2,966,513 and Japanese Patent Publication No. 12931/60, in which molecular oxygen is used for the oxidation in the presence of heavy metal compounds and bromide as catalyst. In these processes, however, the oxidation is carried out under extremely high temperature and high pressure and the reaction is continued under the same severe conditions even after the absorption of oxygen has been stopped, requiring very great care to the safety of operation (and, particularly, to the explosion limit of the gas phase). In this sense, the processes using such severe reaction conditions are disadvantageous from an industrial viewpoint.
U.S. Pat. No. 2,578,759 describes another process of oxidizing acenaphthene with molecular oxygen in the presence of a catalyst of a heavy metal compound and an accelerator.
In Example 1 of this Patent specification, there is an example wherein 110 parts of acenaphthene are oxidized in 1500 parts of propionic acid which is mixed with 108 parts of cobalt acetate and 375 parts of butyl aldehyde, thereby giving naphthalic acid. In this process, the cobalt acetate is used in a large amount, i.e. in a quantity equimolar to the acenaphthene, causing problems encountered in recovery and regeneration of the cobalt catalyst.
As will be seen from the above, the prior art techniques of oxidizing acenaphthene have important defects.
On the other hand, there are known several processes of oxidizing substituted acenaphthenes including, aside from the afore-mentioned processes using bichromates, etc., as oxidizing agents, processes such as described in Japanese provisional publications Nos. 142543/75 and 142544/75, in which molecular oxygen is used for oxidation in the presence of heavy metal compounds and bromides. In these processes, however, it is essential to use the starting substituted acenaphthenes in very dilute concentrations (e.g. when 5,6-dichloroacenaphthene is used, its concentration in the reaction solution must be below 0.045 moles/l) so as to attain good yield. In all the Examples concerning the production of 4,5-dichloronaphthalic acid, the yield of 4,5-dichloronaphthalic acid is below 4.36 g per liter of the reaction vessel. Similarly, in all the Examples concerning the production of 4-sulfonaphthalic acid, the yield per liter of the reaction vessel is below 9.66 g. In addition, the yield of naphthlic acid per liter of the vessel is below 10.3 g as clearly described in Examples of Japanese provisional publication No. 142544/75. Thus, it seems very difficult to reduce the processes into practice because of their poor productive capacity.
In the previously mentioned U.S. Pat. No. 2,578,759, there is no description concerning the oxidation of acenaphthenes substituted with a halogen atom, a sulfo group or a nitro group. Presumably, this is because, in the oxidation of acenaphthenes substituted with a halogen atom, a sulfo group or a nitro group, side reactions such as condensation tend to take place and the oxidation reaction is impeded to a considerable extent as compared with that of unsubstituted acenaphthenes partly due to the fact that starting substituted acenaphthenes per se and intermediates thereof produced during the oxidation reaction are generally low in solubility in organic solvents (and particularly, in lower fatty acids).
When we made an experiment in which an acenaphthene derivative, e.g. 5,6-dichloroacenaphthene, was oxidized in accordance with the procedure of Example 1 of U.S. Pat. No. 2,578,759 which procedure was proposed to oxidize unsubstituted acenaphthene, we found that, since the amount of cobalt acetate used as the catalyst was so great, the reaction conditions became severe, a large amount of a condensation product of 5,6-dichloroacenaphthene was secondarily produced and the oxidation reaction stopped on the way, so that satisfactory results could not be obtained with regard to yield and purity.
As will be understood from the foregoing, the prior art processes of oxidizing acenaphthene or substituted acenaphthenes into corresponding naphthalic acids require very severe reaction conditions such as high temperature and high pressure or the use of very large amounts of heavy metal compounds, or result in poor conversion of acenaphthenes.